Starts to get a bit philosophical on what constitutes "portable" but JIT compilers would emit an opcode based off of whatever the frontend/IR is saying to do surely?
That assumes no per-platform optimisation, which most JIT compilers will do. I can only speak to the dotnet platform as that's what I am using the most at the moment, but its JIT will spot certain functions, like Vector512.LoadAligned and replace it with the the SIMD equivalent. So, it's not just IR op-code to CPU op-code, it's looking for patterns-of-code that can be made more efficient at JIT compile-time.
